Magnetoresistance (MR) is the property of a material to change its electrical resistance in the presence of a magnetic field. Magnetic sensors based on the MR effect can measure the strengths of magnetic fields or the relative direction of the fields. One of the most significant applications of MR sensors is their incorporation into read heads for magnetic recording devices. Five distinct types of MR are ordinary magnetoresistance (OMR), anisotropic magnetoresistance (AMR), giant magnetoresistance (GMR), tunneling magnetoresistance (TMR) and colossal magnetoresistance (CMR).
There are two types of GMR geometries, which are named current in plane (CIP) and current perpendicular to the plane (CPP). Each geometry has a different quantum mechanical effect that changes the probability of conduction electrons scattering throughout their layers and consequently changing the electrical resistance of the GMR material. CIP GMR describes a geometry whereby the magnetic field must be applied in the same direction as the current flow for a change in resistance to be observed in the material and the change in the mean free path of conduction electrons determines the amount of resistance change. Alternatively, CPP GMR refers to a geometry typically having multilayers in high aspect ratio configurations. With CPP GMR, a magnetic field must be applied perpendicular to the current flow for resistance changes to be observed. The magnetic field alters the magnitude of the spin-flip diffusion length of conduction electrons, influencing the resistance change of the GMR material. CPP GMR performance is partially a function of the number of alternating material layers with a higher number of layers giving higher changes in resistances in magnetic fields.